Spontaneous Wetting Dynamics in Perylene Diimide n-Type Thin Films Deposited at Room Temperature by Supersonic Molecular Beam
2016
Chiarella, Fabio | Chianese, Federico | Barra, Mario | Parlato, Loredana | Toccoli, Tullio | Cassinese, Antonio
During the last few years, the perylene tetracarboxylic diimide derivative PDIF-CN₂ has been identified as one of the most interesting compounds to achieve air-stable n-type organic thin-film transistors. In this work, a detailed investigation on the properties of PDIF-CN₂ thin films deposited by supersonic molecular beam deposition technique on HMDS-treated Si/SiO₂ surfaces is reported. Our efforts have been addressed to analyze deeply the role played by the growth rate (R) on the thin film properties, working with the substrates kept at room temperature and using a kinetic energy of the impinging molecules of about 17 eV. In this way, we could observe that while, just after the deposition, the layers were not compact, being composed of small and mostly separated islands with a rate-dependent density, an unconventional spreading effect of the condensate took place over time until the formation of a continuous film. According to our results, this spontaneous wetting process occurs over very long time scales (i.e., from days to months) typical of a viscous phase and exhibits a dynamics also related to the initial growth rate. These remarkable morphological transitions are coherently accompanied by a several orders of magnitude increase in the field-effect charge carrier mobility, achieving maximum values of about 0.1 cm² V–¹s–¹ in a specific R range. We have also found that a 1 h postannealing procedure at only 50 °C guarantees the rapid complete substrate wetting and the structural reassembly of the investigated films, providing comparable high mobility values.
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